mmg_233_2013_genetics_genomicswikiaorg-20200214-history
Lactic acid production deficient Streptococcus mutans
Dental caries is a bacteria-associated infection that causes demineralization and destruction of the hard tissue of teeth 5. Streptococcus mutans is a Gram-positive anerobic bacteria that is the major etiological agent of dental caries. S. mutans ''is able to degrade sucrose into glucose and fructose in the mouth 2. The glucose then form polymer dextran and it will allow ''S. mutans to cohere. S. mutans can utilize the residual sugar in the mouth and degrade them to lactic acid by the enzyme lactate dehydrogenase (LDH) 2. The lactic acid will cause the destruction of the enamel, which consists of hydroxyapatite, and thus cause the formation of dental caries 2. Hillman'' et al.'' constructed a genetically modified strain of S. mutans that lacks the ability of producing lactic acid due to a mutation of the lactate dehydrogenase gene [3 ]. Moreover, this strain can produce antibiotics, giving it more selective advantages in competing with indigenous S. mutans. This lactate production deletion strain can be used as a therapeutic agent to compete with the wild-type strain in human mouth and prevent the formation of mouth caries. This strain has been proved effective in a rat dental model 1. Genetic modifications The deletion of the ldh gene in S. mutans is constructed by homologous recombination. The ldh-deletion has been proved to be a lethal mutation, so the alternative approach is to substitue the'' ldh gene with an alchool dehydrogenase gene to compensate for the ldh'' activity [4 ] . Briefly, a suicide vector harboring an alcohol dehydrogenase gene (adh) from Zymomonas mobilis is transformed into the S. mutans parent strain. The'' ldh gene is substituted by Z. mobilis adh gene via a double crossover and the clone carrying this mutation is screened by counter selection 1. Results of enzyme activity assay and southern blot confirmed both the phenotype and the genotype of the constructed lactate production mutation strain. Besideshe host strain in this research is a spontaneous mutant that expresses two- to three-fold increased amount of mutacin 1140, a broad-spectrum lantibiotic peptide that enhances the competitive strength of the mutant strain during colonization [1 ]. The goal and current status of the research The goal of constructing a lactate production deficient ''S. mutans strain is to utilize the mutant strain as a therapeutic agent. The theraputic approach of utlizing the non-pathogenic mutant strain to replace the wilde type pathogenic microbe is called replacement therapy 5. Upon inoculation into recipients’ mouths, the genetically modified S. mutans is able to compete with the wild type counterpart and other pathogenic microbes, thus decreasing the risk of mouth caries 2. The transgenic strain of S. mutans has been tested on SD rat model and appears to be safe for human clinical trial 3. References 1. Hillman JD, Brooks T, Michalek SM, Harmon CC & Snoep JL (2000) [http://iai.asm.org/content/68/2/543.long Construction and characterization of an effector strain of Streptococcus mutans for replacement therapy of dental caries.] J. Dent. Res. 68: 543–549. 2. Hillman JD (August 2002). "[http://www.ncbi.nlm.nih.gov/pubmed/12369203 Genetically modified Streptococcus mutans for the prevention of dental caries"] . Antonie Van Leeuwenhoek 82 (1-4): 361–6. 3. Hillman JD, Mo J, McDonell E, Cvitkovitch D, Hillman CH (May 2007). "Modification of an effector strain for replacement therapy of dental caries to enable clinical safety trials ". J. Appl. Microbiol. 102 (5): 1209–19. 4. 1. Hillman, J. D., Chen, a, Duncan, M. & Lee, S. W. Evidence that L-(+)-lactate dehydrogenase deficiency is lethal in Streptococcus mutans. Infect. Immun. 62, 60–4 (1994). 5. http://en.wikipedia.org/wiki/Dental_caries